1. Field of the Invention
The present disclosure relates to methods for the synthesis of single-wall carbon nanotubes (hereinafter “SWCNT”) at lower temperatures than previously reported for synthesis routes utilizing methane as the feedstock and Fe:Mo compositions as the catalyst.
Although significant progresses have been made in SWCNT synthesis, knowledge about the nucleation and/or growth mechanism, which could lead to type-selective growth and/or large scale production, is still very limited. Early on, it was believed that very high temperature conditions, for instance, 1000-2000° C., were necessary for SWCNT synthesis, since arc discharge and laser ablation methods were mainly used. With the advent of chemical vapor deposition (hereinafter “CVD”) methods, this opinion changed, and the SWCNT growth temperatures keep decreasing.
2. Discussion of the Related Art
Maruyama et al. (Maruyama, S.; Kojima, R.; Miyauchi, Y.; Chiashi, S.; Kohno, M. Chem. Phys. Lett. 2002, 360, 229) reported SWCNT growth at 550° C. by using methanol as carbon feedstock and Fe/Co as catalyst. Bae et al. (Bae, E. J.; Min, Y-S.; Kang, D.; Ko, J-H.; Park, W. Chem. Mater. 2005, 17, 5141) reported synthesis at 400° C. by using a plasma enhanced CVD method; and recently Cantoro et al. (Cantoro, M.; Hofmann, S.; Pisana, S.; Scardaci, V.; Parvez, A.; Ducati, C.; Ferrari, A. C.; Blackburn, A. M.; Wang, K-Y.; Robertson, J. Nano Lett. 2006, 6, 1107) further reduced the synthesis temperature to 350° C. In most cases these reported synthetic routes utilized exothermic carbon-containing feedstocks.
Theoretical analysis and models indicate that there are three relatively independent stages for the growth of a SWCNT on a catalyst:
1) catalytic decomposition of carbon feedstock gas yielding C atoms,
2) diffusion of these C atoms to the tube end that is strongly attached to the catalyst surface, and
3) incorporation of the C into the tube wall. (Ding. F.; Larsson, P.; Larsson, J. A.; Ahuja, R.; Duan, H.; Rosén, A.; Bolton, K. Nano Lett. 2008, 8, 463, and Ding. F.; Rosén, A.; Bolton, K. J. Phys. Chem. B 2004, 108, 17369.)
Thermal activation is necessary for each one of these stages and thus, there must be a characteristic threshold temperature for each step. Therefore, we define:                Tdec as the lowest temperature sufficient to decompose the feedstock;        Tdiff as the temperature above which the C atoms can diffuse to the SWCNT end at a reasonably high rate; and        Tg as the temperature at which C atoms can be readily incorporated into the tube wall.        
While the interplay of these stages can be complex, the highest temperature of these three threshold temperatures should be the overall threshold temperature of the apparent SWCNT growth. Recent density functional calculations show that the diffusion barrier of a C atom on catalyst surface is only about 0.5 eV (for example, the diffusion barrier of a C atom on a Ni (111) surface) (Abild-Pedersen, F.; Nørskov, J. K.; Rostrup-Nielsen, J. R.; Sehested, J.; Helveg, S. Phys. Rev. B 2006, 73, 115419) so the Tdiff may be as low as room temperature. The feedstock decomposition temperature sensitivity appears to depend upon, at least, the type and size of the catalyst particle, the type of the chemical reaction, and the feedstock itself. Finally, incorporation of C atoms into the SWCNT is apparently feedstock independent, although presently this stage of the process is not well known, and is the subject of further investigation.
While it remains difficult to determine the dominant step influencing the SWCNT growth temperature, we note that the reported low temperature SWCNT growths were achieved in synthesis routes using active hydrocarbon sources with exothermic decomposition only (for instance, C2H4→2C+2H2 ΔH°=−38.2 kJ/mol and C2H2→2C+H2, ΔH°=−224.3 kJ/mol at 800° C.) (David R. Lide, ed. CRC Handbook of Chemistry and Physics (87th Edition); Taylor and Francis: Boca Raton, Fla., 2007) or with decomposition obtained with the assistance of plasma treatment of the feedstock. These results would seem to suggest that the decomposition of carbon feedstock limits SWCNT growth. If that is true, then the threshold temperature for SWCNT growth with less active endothermic carbon feedstock must be higher than the threshold temperature for SWCNT growth when more active exothermic carbon feedstock are utilized.